What happens to the force between the spheres when you increase the mass of one of the spheres?
what happens to the force between the spheres when you increase the mass of both spheres?

The force between the spheres increases when the mass increases in one of the spheres.

Explanation:

            Newton law of universal gravity extends gravity beyond the earth's surface. This gravity depends directly on the mass of both objects and is inversely proportional to square of distance between their centers.  

          Since gravity is directly proportional to “mass of both interacting objects”, stronger objects with greater gravitational force attract. If the mass of one object increases, gravity between them also increases. For example, if an object's mass of one double, force between them also doubles.  

The force between two spheres increase when mass of one or both spheres are increased.

Explanation:

According to universal law of gravitation “every body in the universe attracts every other body with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them”.

It is given by the expression

Where G is the gravitational constant, are the masses of the bodies and  r is the distance between them.

In the question two conditions are given.

Since force between two objects is directly proportional to the masses, the force increases in both cases.